1. Field of the Invention
The present invention relates to a liquid crystal display device, and more in particular to alignment treatment technique of alignment layers of a liquid crystal display device.
2. Description of the Related Art
A liquid crystal display device comprises a pair of opposed and spaced substrates, an electrode and an alignment layer formed on one of the substrates, an electrode and an alignment layer formed on the other substrate, and a liquid crystal inserted between the pair of the substrates. The electrode on one of the substrates is a common electrode, and the electrode on the other substrate is formed as pixel electrodes. The pixel electrodes can be arranged with an active matrix. Also, the electrode may be arranged only on one of the substrates (in IPS mode, for example). A black matrix and a color filter are arranged on one of the substrates.
In the conventional TN liquid crystal display device, the alignment layer is rubbed so that the liquid crystal aligns in a predetermined direction. The rubbing is a treatment to rub the alignment layer with a cloth as Rayon, and dust occurs as the cloth of Rayon or the like is brought into the clean room. Also, the rubbing generates static electricity which may destroy the TFT (thin film transistors) of the active matrix. In view of this, JP-A-9-354940 which is an application filed prior to the present application proposes to carry out the alignment treatment by the irradiation of ultraviolet light. The alignment treatment with the irradiation of ultraviolet light can obviate the problem of rubbing. According to that proposal, the unpolarized ultraviolet light is irradiated on the alignment layer having a vertical alignment property, and the alignment layer is aligned so that the alkyl side chain realizing the vertical alignment is only partly destroyed. In the case of a horizontal alignment layer, however, the alignment treatment by irradiation of the ultraviolet light is difficult to realize.
In the case where the alignment layer having the vertical alignment property is treated to realize alignment by irradiating the ultraviolet light, the degree of the alignment depends on the exposure energy of the ultraviolet light to a large measure. An examination shows that it is desirable to irradiate the ultraviolet light of a proper exposure energy. In the case where the exposure energy is smaller than the proper amount, for example, the pretilt angle established by the alignment layer is high (nearer to the normal to the substrate surface). When distributing spacers for maintaining the cell gap of the liquid crystal panel, therefore, the alignment of the portion around the spacers is disturbed. The result is a display fault with an alignment in which a black spot is generated around the spacers at the time of display.
In the case where the exposure energy is larger than the proper amount, in contrast, the pretilt angle established by the alignment layer is low. The excessive exposure energy, however, may make a horizontally-aligned portion and a portion with a filling streak caused at the time of filling liquid crystal, thus leaving a trace. Another problem is that a desired vertical alignment cannot be obtained but a horizontal alignment develops.
Also, the ultraviolet light cannot be irradiated on the surface of the alignment layer at any angle. In the case where the ultraviolet light is irradiated at an angle almost perpendicular to the surface of the alignment layer, for example, it is impossible to selectively leave only a portion of the alkyl side chain of the alignment layer and hence impossible to obtain an alignment in the desired direction.